Generally, a play (backlash) is provided between gears in order to obtain a smooth operation in gear meshing.
A worm wheel deceleration mechanism of the electric power steering apparatus also requires a proper backlash. Normally, an axis-to-axis distance between a gear housing shaft of a housing and an output shaft of a motor is set to the same as an axis-to-axis distance (a worm gear working radius plus a wheel gear working radius), and hence a backlash derived from a variety of working scatters in addition to the preset backlash. This caused-by-the-scatter backlash gives a driver an uncomfortable feeling due to tooth-butting noises when a vehicle travels on a rough road, resulting in a decline of value of a commercial product.
A countermeasure therefor was that the tooth-butting noises are reduced by setting the backlash as small as possible while raising a gear accuracy, or by a method, as disclosed in Japanese Patent Application Laid-Open No. 11-43062, of absorbing vibrations in a way that provides an elastic body between a worm shaft and a bearing thereof, and so forth.
Moreover, Japanese Patent Application Laid-Open No. 10-281235 discloses a power transmission apparatus, wherein the worm shaft is supported through the bearing in a shaft hole taking an elliptic shape that is formed in a worm shaft housing portion and is eccentric on the side of the worm wheel, and O-ring shaped elastic member is provided in a circular groove formed concentrically in an inner peripheral surface of the shaft hole, this elastic member biases the bearing (the worm shaft) towards the worm wheel, thereby eliminating the backlash.
Further, when assembling the worm shaft and the worm wheel, there might occurs a dimensional error between the worm, the worm shaft, a bearing portion supporting the worm shaft, the worm wheel and a steering shaft for supporting the worm wheel, etc. It follows that the backlash is caused at a comparatively large rate due to this dimensional error after being assembled. It was therefore required that the parts be assembled separately according to the accuracy thereof. Further, if a higher output of an auxiliary steering force advances as seen over the recent years, this results in an increase in abrasions of teeth of the worm and of the worm wheel, and a drawback comes to appear, wherein the occurrence of the backlash can not be avoided.
A known method for preventing gear butting noises derived from these causes is a method of eliminating the backlash by applying a pre-load to the worm towards the worm wheel. For example, as disclosed in Japanese Patent Application Laid-Open Nos. 2001-322554 and 2001-108025, there are known methods of generating a pre-load force by causing a deformation of an elastic body provided between an outer ring of the bearing provided at a side end of the worm and a gear housing.
Of the conventional electric power steering apparatuses disclosed in the former Publications, the apparatus disclosed in Japanese Patent Application Laid-Open No. 10-281235 has a drawback that the worm shaft housing portion is easy to abrade when the bearing moves and a problem that the worm shaft is easy to cause an axis deviation with respect to the motor shaft when the worm shaft is biased by the elastic member, and so on.
Generally, as far as there exists the backlash depending on a condition of a rough road, a difference between inputs from a motor vehicle and so on, a problem is that the tooth-butting noises can not be completely muffled, and there is a necessity of reducing the tooth-butting noises for every motor vehicle.
In the conventional electric power steering apparatuses disclosed in the latter Publications, a problem is that the quantity of deformation of the elastic body is determined from an outside diameter of a rolling bearing that is determined based on an inside diameter of a housing and a load, the elastic body has no alternative but minutely deforms because of a restraint in terms of a space, besides a pre-load force for the worm largely changes due to a minute displacement of a worm end that is caused by a scatter in working of the gear housing and by a deflection in meshing, and it is therefore difficult to ensure the pre-load force expected.
If this pre-load force is too large, an operating force declines, which brings about deterioration in feeling when neutral of steering. Whereas if too small, the gear butting noises emit, and an essential purpose can not be attained.
Thus, according to the prior art, even in a case where an axis-to-axis distance between the worm and the worm wheel changed due to the gear abrasion, etc., the pre-load varies due to the minute displacement of the worm, and it was difficult to ensure the stable pre-load.
Moreover, the load and a rotational torque acting in radial directions are applied to the elastic body provided on the outer periphery of the bearing whenever steered, and hence, as disclosed in Japanese Patent Application Laid-Open No. 2001-270448, deterioration such as a fatigue, etc occurs in the elastic body according to a structure for making flexural the elastic body serving as a slide bearing, and a problem arises from this deterioration, wherein the quantity of backlash rises, and the pre-load force decreases from a permanent deformation of the elastic body.
The load and the rotational torque acting in the radial directions have a great influence on the pre-load force given by the elastic body, with the result that the quantity of deformation of the elastic body augments. Therefore, if the axis-to-axis distance between the worm and the worm wheel increases and if a meshing area between the gears decreases, there also arises a problem that a strength of the gear declines this time.
An object of the present invention lies in providing a simply-structured electric power steering apparatus capable of improving the drawbacks to the examples of the prior art described above, eliminating an existence of the backlash and reducing tooth-butting noises without any decline of a power transmitting performance.